A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a stable and well-regulated output, switched-mode power converters are frequently employed for an advantage. Switched-mode power converters generally include an inverter, a transformer having a primary winding coupled to the inverter, an output rectifier coupled to a secondary winding of the transformer, an output filter and a controller. The inverter generally includes a power switch, such as a field-effect transistor (FET), that converts an input voltage to a switched voltage that is applied across the transformer. The transformer may transform the voltage to another value and the output circuit generates a desired voltage at the output of the converter. The output filter typically includes an inductor and an output capacitor. The output capacitor smooths and filters the output voltage for delivery to a load.
In many power converter applications, the output voltage requirements and therefore the voltage handling requirements of the power switch, are large. In a conventional silicon semiconductor wafer, large voltage handling capability is difficult to achieve in a laterally constructed FET due to the inherently close proximity of the source and drain. This arrangement thereby causes lower than desired values of breakdown voltage for the device. This has often necessitated the use of a power switch called a vertical device metal oxide semiconductor FET (VDMOSFET). The VDMOSFET is constructed such that the drain is positioned on the bottom of the device, and the source is positioned on the top, with the gate vertically interposed between the drain and source. This vertical arrangement allows the VDMOSFET to achieve a larger breakdown voltage, and therefore, allows the VDMOSFET to accommodate a larger operating voltage while using conventional silicon semiconductor wafer technology.
Unfortunately, the VDMOSFET has a greater intrinsic on-resistance, which becomes important when the VDMOSFET is used as a switch, and also possesses a greater intrinsic capacitance. The greater on-resistance and capacitance are due, in part, to the increased separation of the source and drain and the added layers needed to obtain the larger breakdown and operating voltage capability. The greater on-resistance of the VDMOSFET increases the losses contributed by the VDMOSFET and may therefore reduce an overall efficiency of a power converter employing the VDMOSFET. Additionally, the added capacitance decreases switching speed and therefore may increase switching losses as well.
Another problem arises from the general trend of such electronic devices toward ever smaller device sizes and ever greater packing density. As the VDMOSFET's size continues to shrink and device packing density increases, the junction field effect transistor resistance of the vertical region between the two adjacent P wells also increase, thereby inhibiting the performance of the device even further. Thus, the VDMOSFET's use in such power converters may be substantially limited in the near future due to these physical limitations.
Accordingly, what is needed in the art is a MOSFET that provides an advantageous breakdown voltage characteristic while exhibiting a low on-resistance as a switch.